The present invention relates generally to a method and apparatus for removing coatings from a workpiece or substrate, and more particularly to a method and apparatus for removing such coatings in a controlled sequence of steps.
Some types of workpieces and parts require that one or more layers of coatings be applied to the workpiece to protect the underlying base metal from heat, abrasion, and other elements encountered by the workpiece in its normal usage. For example, turbine blades and other turbine parts may be formed of a base metal, a bond coating that is often diffusion bonded metallurgically to the base metal, and an outer thermal barrier coating (TBC) applied to the bond coat. As is well known, the composition of the TBC (e.g. ceramic) makes it relatively easy to remove using manual grinding, dry grit blasting, chemical stripping and water jet stripping. However, as set forth in greater detail in U.S. Pat. No. 6,905,396, which is incorporated herein by reference, all of these known manually applied removal methods have various drawbacks in that they are not particularly effective in removing the more difficult bond coat, and, in the case of acid or chemical stripping, environmental problems are encountered and there is a tendency to damage the underlying base metal as part of the removal process.
The invention described in the aforesaid '396 patent provides for a controlled removal method that can be carried out using an abrasive water jet that is mechanically, rather than manually, operated to move relative to the workpiece and thereby remove coatings from the base metal as the abrasive water jet is passed relative to the surface of the workpiece. However, even though the relative movement of the abrasive water jet across the workpiece surface is controlled, the abrasive water jet itself generally removes substantially equal amounts of the coatings during each pass of the abrasive water jet, and the abrasive water jet is not controlled in a way that would properly compensate for variations in the thickness of the TBC caused by the use of the workpiece, such as a turbine blade. Additionally, because of variations in the overall thickness of the coatings to be removed, the relatively constant flow of the abrasive jet would sometimes remove more or less of the base metal than was desired.
Accordingly, a need exists for a method of removing layers of coatings in a controlled manner that compensates for variations in the overall thickness of the coatings. A need also exists for such a method that is controlled in a way that it removes a desired small amount of the base metal, such as the diffusion layer formed when a bond coat is diffusion bonded to the base metal, to provide a clean, pristine surface on the base metal after the coatings have been removed so that new coatings can be applied to the base metal.